The present invention relates to an alloy material excellent in both the corrosion resistance and abrasion resistance, a method for producing the same, a material for use in the production of the same, a metallic member using the same and a method for improving the corrosion resistance and abrasion resistance of the surface of said metallic member.
In general, in order to improve the corrosion resistance and abrasion resistance of metallic members constituting machinery, the surface of the metallic members has hitherto been modified, for example, by hardening or plating materials for constituting the metallic members, such as stainless steel.
Besides the above-described surface treatment method, various proposals have been made in recent years off a method wherein particular hard particles are added to metallic materials for constituting metallic members. Examples of such methods include a method as described in Japanese Patent Laid-Open Publication No. 181470/1983, wherein hard particles of WC, NbC, TiC, CrC, VC, etc. are added in a matrix metal comprising a Ni-base, Co-base or Fe-base alloy or to the surface of the matrix to improve the hardness or abrasion resistance of the matrix metal. The present inventors have found that although this method is effective to some extent in improving the abrasion resistance, it is not always satisfactory because the toughness of the alloy is disadvantageously lowered. For example, hard particles, such as NbC (melting point: 3480.degree. C.) and TiC (melting point: 3180.degree. C.), have a high melting point and are hard to melt, so that it is difficult to avoid the addition of the hard particles in an unmelted state. For this reason, there occurs a problem that, in the step of forming a corrosion-resistant and abrasion-resistant layer, the hard particles remain in a relatively large form in the matrix metal, which unfavorably gives rise to a lowering in the toughness of the metallic material.
Also, melting of a carbide as the additive in the matrix metal often leads to embrittlement of materials. For example, it is known that, when MoC or WC is melted together with a Ni-base, Co-base or Fe-base alloy, a brittle (fragile) M.sub.6 C compound crystallizes. The crystallization of the brittle compound does not contribute to an improvement in the abrasion resistance and further results in embrittlement of the matrix metal.
Japanese Patent Laid-Open Publication No. 157796/1988 discloses a method for producing a roll for hot rolling. This method is directed toward the production of a roll having abrasion resistance and cracking resistance and being excellent in the toughness by adding a VC powder to an iron-base alloy, and the addition of a VC powder in an amount of 10% by weight or more to an iron-base alloy is essential to this method. In this method, however, an improvement in the hardenability is contemplated. According to the knowledge of the present inventors, when an Fe-base alloy is used as a parent metal, the addition of VC to the parent metal unavoidably gives rise to the formation of large amounts of a plurality of carbides, such as WC and CrC, so that the effect contemplated in the present invention is not always attained. Further, this method is not a technique for recrystallizing or reprecipitating VC in a particular state and is not always effective in improving both the abrasion resistance and corrosion resistance.
Further, in the conventional method, when hard particles are added, the temperature rise should be limited to the minimized degree necessary for the deposition of the matrix metal for the purpose of inhibiting the crystallization of the above-described fragile compounds. Further, in order to attain this purpose, it is necessary to increase the particle diameter of hard particles to be added to some extent. An increase in the particle diameter, however, makes it difficult to attain a homogeneous dispersion, which results in a lowering in the toughness of the metallic material. As described above, the presence of coarse hard particles in a heterogeneous distribution in the matrix metal causes these hard particles to easily fall off when they are subjected to frictional movement.
The above-described problems of heterogeneous distribution of hard particles and falling-off of hard particles attributable to the heterogeneous distribution of the hard particles are accelerated also by a difference in the specific gravity between the matrix metal and the hard particles. For example, the possibility of occurrence of aggregation or heterogeneous distribution of the hard particles becomes particularly high when an Fe-base alloy is used as the matrix metal with Al.sub.2 O.sub.3, SiC or TiC being used as the hard particle. Further, when a Cr-base carbide is used as the hard particle, a carbide of Cr recrystallized or reprecipitated in the step of cooling after heating aggregates and grows into a coarse carbide particle, which leads to a problem that the hard particles become liable to fall off from the matrix metal.
International Publication WO91/09980 discloses a method for producing an alloy having excellent resistance to corrosion and abrasion, which comprises subjecting a powder mixture comprising a metallic powder of at least one alloy selected from Fe-base, Co-base and Ni-base alloys and a vanadium carbide powder having a particle diameter of 60 to 100 .mu.m to melting and/or repetition of re-melting with a heat source having a high energy density. The use of the powder having a relatively large particle diameter has an advantage of facilitating the alloying. In this method, however, although fine vanadium carbide particles having a diameter of 10 .mu.m or less crystallize and precipitate, since some vanadium carbide particles remain unmelted, the troublesome melting step should be repeated many times to melt the vanadium carbide particles remaining unmelted. Further, observation under a microscope by the present inventors has revealed that part of the unmelted vanadium carbide is present in a massive form nevertheless, so that a fine vanadium carbide particle phase heterogeneously distributes, which unfavorably makes it impossible to attain an intended abrasion resistance and often causes occurrences of cracking and peeling due to local breaking.